It is known that molybdenum trioxide is considered to be the most important molybdenum compound. In commerce, three common grades of MoO.sub.3 are the Technical Grade (approximately 90% or more MoO.sub.3), Grade B (approximately 98% MoO.sub.3) and Pure Grade (approximately 99.9% MoO.sub.3).
The reduction of molybdic oxide (molybdenum trioxide MoO.sub.3) to metallic molybdenum has been the subject of considerable investigation. For example, in the November 1964 Journal of Metals, A. B. Michael and J. B. Hanway, Jr. pointed out the following:
"The hydrogen reduction of molybdic oxide has been demonstrated to occur in stages. During the reduction, molybdic oxide successively passes through several lower oxides and eventually metallic molybdenum is produced. The temperatures required for practical degrees of production progressively increase as the lower oxides are formed. For simplicity, however, the reduction may be considered to take place in two stages: (1) molybdic oxide (MoO.sub.3) is reduced to molybdenum dioxide (MoO.sub.2) at a temperature of approximately 500.degree. C., and (2) molybdenum dioxide (MoO.sub.2) is reduced to molybdenum metal at temperatures as low as 750.degree. C.; a more practical temperature for the final state of reduction, however, is about 1000.degree. to 1100.degree. C.
The authors then proceeded to describe their development and testing of a single-stage fluid bed process for converting MoO.sub.3 to Mo metal. Their process sought to retain the heat generated in the exothermic first stage of reduction within the reactor so that heat required to preheat the fluidizing hydrogen to accomplish the endothermic second stage of reduction would be kept within practical limits. It was postulated that the MoO.sub.3 fed to the reactor would become molten enough to attach itself to the original bed particles before or while being reduced to the dioxide. It was considered this would result in gradual buildup or growth of bed particles so that the final molybdenum product would be granular. Michael et al. found an operating temperature in their single-stage bed approaching 955.degree. C. was preferred. It is known, however, that at temperatures above 650.degree. C., MoO.sub.3 will sublime causing the bed to get sticky and eventually defluidize. U.S. Pat. Nos. 2,398,114; 2,987,932; 3,264,098; 3,865,573 and 4,045,216 can also be mentioned. In U.S. Pat. No. 2,398,114, a tube-and-boat furnace was used and batches of water-granulated MoO.sub.3 were treated therein stage-wise with the first stage being conducted at a temperature not substantially exceeding 630.degree. C. in an atmosphere of dilute reducing gas which could be hydrogen, carbon monoxide, ammonia or mixtures with sufficient dilution of the reducing gas with dilutents such as steam, nitrogen, or carbon dioxide to control temperature rise in the exothermic first stage. The second stage reduction to molybdenum metal was then conducted in hydrogen at the higher temperature of about 1040.degree. C. U.S. Pat. No. 2,987,392 is directed to the reduction of MoO.sub.3 to molybdenum metal in a boat-and-tube furnace using gaseous reductants such as hydrogen, ammonia, carbon monoxide, various hydrocarbons, manufactured atmospheres ("endogas"), metallic vapors and mixtures. It was considered that gaseous reductants were not effective with significant depth in the bed, hence a solid reductant such as hexamine was mixed with the bed material. It was contemplated that the entire reduction to metal could be accomplished using only the solid reductant. U.S. Pat. No. 3,264,098 is directed to reduction of MoO.sub.3 to molybdenum metal in a fluid bed which can be either single-stage or multi-stage using hydrogen as the reducing gas. U.S. Pat. No. 4,045,216 is directed to a continuous process for producing a dense pelletized metallic molybdenum product from pelletized molybdenum trioxide feed material in a vertical reactor using hydrogen as the principal reducing agent wherein, in a first stage molybdenum trioxide is reduced to molybdenum dioxide at preferably 600.degree. to 640.degree. C. in hydrogen which is diluted with nitrogen and water vapor and the second stage reduction of molybdenum dioxide to molybdenum is conducted at a temperature exceeding 900.degree. C. using a gas richer in hydrogen than that used in the first stage. U.S. Pat. No. 3,865,573 is directed to the stepwise reduction of molybdenum trioxide to molybdenum dioxide at 500.degree.-600.degree. C. followed by reduction of the dioxide to metal at 800.degree.-900.degree. C. Hydrogen, reformed gas or cracked ammonia are used as the reducing gas. Feed for the process is briquetted with iron or iron oxide powder to provide a metallized ferromolybdenum briquette for addition to molten steel. The patent notes that impurities merely pass through the process. Patent application Ser. No. 603,392, filed Apr. 24, 1984 (AMAX No. 1095) is directed to the reduction of molybdenum trioxide to molybdenum dioxide in a rotary kiln using ammonia as a reductant at a temperature of 400.degree. to 500.degree. C.
The art thus recognizes that the reduction of MoO.sub.3 to Mo metal is preferably conducted in stages to yield MoO.sub.2 as the intermediate product, with separately controlled atmospheres and temperatures for each stage and using various processing procedures including reactors handling briquetted feed, the rotary kiln and the fluid bed. Both single stage and multi-stage operation are contemplated as well as the use of both static and moving beds of material.
There is no mention in this art of utilizing the reduction process itself to reduce the impurity content of molybdenum. It is known that commercial molybdenum products contain impurities at various levels. These impurities can include iron, aluminum, silicon, lead, tin, copper, zinc and bismuth. Depending upon the desired commercial application for the molybdenum product, such impurities can be more or less harmful. Reductions in the content of at least some of the impurities, including particularly lead, copper, zinc and bismuth would improve the marketability of the molybdenum product having such reduced impurity content.
It is to be recalled in this connection that existing means for removing impurities from molybdenum products are complicated and expensive, since such means involve dissolving the oxide or metal powder, solvent extraction, ion exchange, selective precipitation and the like followed by precipitation PG,6 of a molybdenum compound such as ammonium molybdate, conversion to oxide and reduction to metal.
It would be of substantial advantage to provide a method for reducing the impurity content of molybdenum during the course of the process of reducing molybdic oxide to metal and it is to such a process that the present invention is directed. Reference is made to the accompanying drawing which depicts too externally heated fluid bed reactors connected in tandem for carrying out the two stage reduction of molybdenum trioxide in the production of molybdenum metal powder.